IT8922749A1 - METHOD OF SURFACE TREATMENT OF SILICA-COATED PHOSPHORUS PARTICLES FOR THE SCREEN OF A CATHODE RAY TUBE - Google Patents

Description

Description of the patent for an industrial invention entitled:

"Method of surface treatment of silica-coated phosphorus particles for the screen of a cathode ray tube"

SUMMARY

A method of surface treating dry powdered phosphorus particles to control their triboelectric charging characteristics includes the steps of providing the phosphorus particles with a first silica coating, dissolving a coupling agent selected from the group consisting of silanes and titanates in an appropriate solvent to form a mixture, surface coating the silica-coated phosphorus particles with the mixture to provide a second coating of the coupling agent on the phosphorus particles, filtering the surface-treated particles, rinsing the surface-treated particles, and drying the surface-treated particles. The resulting surface-treated, dry powdered phosphorus particles are used to make a luminescence-type display screen (22) for a CRT 10 cathode ray tube. The coupling agent overlying the silica coating controls the triboelectric charge characteristics of the phosphorus particles during the electrophotographic fabrication of the cathode ray tube screen.

DESCRIPTION

The present invention relates to the electrophotographic fabrication of a display screen for a cathode ray tube (CRT) and more specifically to a method of surface treating dry powdered phosphor particles with a coupling agent to control the triboelectric charging characteristics thereof.

A typical cathode ray tube, of the shadow mask or pinhole type, comprises an evacuated bulb containing a viewing screen comprising an array, or matrix, of three-color emitting phosphor elements arranged in a celestial array; means for producing three converging electron beams directed at the viewing screen; and a color-selecting structure, the so-called shadow mask, consisting of a thin sheet of metal with a variety of precisely positioned apertures between the image viewing screen and the means for producing the corresponding electron beams. The apertured sheet of metal masks the screen, while differences in convergence angles allow the transmitted portions of each electron beam to selectively excite the phosphor elements capable of producing luminescence in the desired color. A matrix of light-absorbing material surrounds the various phosphor elements.

In a prior art process, for the formation of each array, or matrix, of phosphor elements on the front display plate of a cathode ray tube, the inner surface of the front plate is coated with a suspension of a photosensitive binder and phosphor particles capable of emitting light in accordance with one of three emission colors. The slurry is then dried to form a coating, while a light field is projected from a source through apertures in the shadow mask, with the light field incident on the dried coating so that the shadow mask can function as a photographic "matrix." The exposed coating is then developed to produce the first-color phosphor elements. The process is repeated for the second-color and third-color phosphor elements, respectively, using the same shadow mask but repositioning the light source for each exposure. Each light source position approximately maps to the convergence angle of one of the electron beams that excites the corresponding color phosphor elements. A more complete description of this process, known as the wet photolithographic process, is given in U.S. Patent No. 5,000,000. 2,625,734 issued to H. B. Law, January 20, 1953.

A disadvantage of the wet process described above is that it cannot meet the higher resolution demands of next-generation entertainment devices, as well as the higher resolution requirements for monitors, workstations, and applications requiring color alphanumeric text. Furthermore, the wet photolithography process (including matrix processing) requires 182 major process steps, necessarily requires extensive piping and clean water, requires the recovery and regeneration of phosphors, and requires significant amounts of electrical energy for exposure and drying of the phosphors.

U.S. Patent 3,475,169, issued to H. G. Lange on October 28, 1969, describes a process for producing a cathode ray tube screen by electrophotography. The inner surface of the cathode ray tube faceplate is coated with a volatilizable conductive material, over which is a further coating of volatilizable photoconductive material. The photoconductive layer is then uniformly charged, selectively exposed to light passing through the shadow mask to establish a latent charge image, and developed with a high molecular weight carrier liquid. The carrier liquid carries suspended particles of phosphor material capable of emitting luminescence of a given color, which are selectively deposited on appropriately charged areas of the photoconductive layer to develop the latent image. The charging, exposure, and deposition process is repeated for each of the phosphor materials capable of emitting the three corresponding colors represented by green luminescent phosphors, blue luminescent phosphors, and red luminescent phosphors, deposited on the screen. An improvement in the electrophotographic realization of screens has been described in the patent.

U.S. Patent 4,448,866, issued to H.G. Olieslagers et al. on May 15, 1984. In accordance with that patent, adhesion of the phosphor particles was increased by uniformly exposing portions of the photoconductive layer between adjacent portions of the deposited phosphor particle pattern with light after each deposition step to reduce, or discharge, any residual charge and permit more uniform charging of the photoconductor for subsequent depositions. Since these latter two patents describe an electrophotographic process that is essentially a wet process, many of the disadvantages previously described with respect to the wet photolithographic process of U.S. Patent 2,625,734 also apply to the previously cited wet electrophotographic process.

U.S. Patent Applications Nos. 287,356 and 287,357, filed by P. Datta et al. on December 21, 1988, respectively, describe an improved process for the fabrication of screen assemblies for cathode ray tubes using triboelectrically charged dry powdered shield structural materials and using surface-treated support beads having a coupling agent thereon to control the polarity and magnitude of the imparted charge.

The Proprietor has determined that although display screens for cathode ray tubes can be fabricated electrophotographically using untreated phosphor particles, surface treatment of the phosphor particles increases the triboelectric charge on the phosphor particles, thereby causing more phosphor particles to attach to each carrier bead. This improves the efficiency of the dry electrophotographic process and increases the screen weight by a factor of about two to about nine times for displays fabricated using surface-treated phosphors.

In accordance with the present invention, a method of surface treating phosphor particles in dry powder form for use in the manufacture of a luminescent display screen for a cathode ray tube for controlling the triboelectric charge characteristics thereof includes the steps of providing the phosphor particles with a first silica coating, dissolving a coupling agent selected from the group consisting of silanes and titanates in an appropriate solvent to form a mixture, coating the surface of the silica-coated phosphor particles with the mixture to form a second coating of the coupling agent on the phosphor particles, filtering the surface-treated particles, rinsing the surface-treated particles with the solvent, and drying the surface-treated particles. The resulting phosphor particles Many, surface-treated, and dry-sprayed materials are used to form the screen. The coupling agent that overlies the silica coating controls the triboelectric charge characteristics of the phosphor particles during the electrophotographic fabrication of the screen.

The present invention will become more apparent from the following detailed description, which should be considered in conjunction with the accompanying drawings, in which:

Figure 1 is a plan view, partly in axial section, of a color cathode ray tube constructed in accordance with the principles of the present invention;

Figure 2 is a cross-section of a tube screen assembly schematized in Figure 1;

Figures 3a-3e illustrate various stages in the manufacturing process of the tube illustrated, schematically, in Figure 1.

Figure 1 illustrates a color cathode ray tube (CRT) having a glass bulb 1 comprising a rectangular faceplate panel 12 and a tubular neck section 14, these parts being connected by a rectangular funnel 15. The funnel section 15 has a conductive inner coating (not shown) in contact with an anode button 16, the conductive inner coating extending into the neck section 14. The panel 12 comprises a faceplate display plate, the substrate 18, and a flange or peripheral sidewall 20 which is sealed to the funnel section 15 by a glass frit 21. A three-color phosphor screen 22 is supported on the inner surface of the faceplate 18. The screen 22, illustrated in Figure 2, is preferably a strip screen which includes a plurality of glass frit 21. of elements consisting of red-emitting, green-emitting, and blue-emitting strips of phosphor material designated R, G, and B, respectively, such strips being arranged in color groups of three strips, or "triads," in cyclic order, said strips extending in a direction generally normal to the plane in which the electron beams are generated. In the normal viewing position for this embodiment, the phosphor material strips extend in a vertical direction. Preferably, the phosphor material strips are separated from each other by a light-absorbent matrix material 23, as is common in the art. Alternatively, the display may be a dot screen. A thin layer of conductive material 24, preferably aluminum, overlies the shield 22 and provides a means for applying a uniform potential to the shield, while this layer is also used to reflect light emitted by the phosphor elements through the faceplate 18. The shield 22 and the overlying aluminum layer 24 form a shield assembly.

Referring again to FIG. 1 of the accompanying drawings, it should be noted that a multi-aperture color-selection electrode, also referred to as a "shadow mask," designated at 25, is mounted, for removal by conventional means, in a predetermined spacing relationship with the screen assembly. An electron gun 26, shown diagrammatically by dotted lines in FIG. 1, is mounted centrally within the neck section 14 of the tube for the purpose of generating and directing three electron beams 28 along converging paths through apertures in the mask 25 to the screen 22. The gun 26 may be, for example, a bipotential electron gun of the type described in U.S. Patent 4,620,133, issued to Morrell et al. on October 28, 1986, or it may be a dual-aperture ... belong to any other appropriate type of electron guns.

Tube 10 is designed for use with a magnetic deflection yoke located therein, such as yoke 30 located in the transition region between funnel portion 15 and neck portion 14. When activated, yoke 30 subjects the three electron beams 28 to appropriate magnetic fields which cause the beams to scan horizontally and vertically to form a rectangular pattern above screen 22. The initial deflection plane (at zero deflection) has been indicated by the line P-P in FIG. 1, located approximately midway through deflection yoke 30. For simplicity, the actual curvatures of the deflection beam paths in the deflection region have not been shown.

Screen 22 is fabricated using the novel electrophotographic process shown schematically in FIGS. 3a-3e, which process has been described in U.S. Patent Application No. 287,356, cited above. Initially, the panel is washed with a caustic solution, rinsed with water, etched with buffered hydrofluoric acid, and rinsed again with water, as is normally done in practice. The inner surface of the front display plate 18 is then coated with a layer 32 of an electrically conductive material, thereby providing an electrode for an overlying photoconductive layer 34. The electrically conductive layer is coated with the photoconductive layer 34 comprising a volatilizable organic polymeric material, an appropriate photoconductive dye, and a solvent. The composition and method of forming the electrically conductive layer 32 and of forming the photoconductive layer 34 have been described in U.S. patent application No. 5,000,000.

287.356.

The photoconductive layer 34 overlying the conductive layer 32 is charged, in a dark environment, by means of a conventional positive-type corona discharge apparatus 36, illustrated schematically in FIG. 3b, which is moved across the layer 34 and charged with a voltage in the range of 200 to 700 volts, with a preferred range being 200 to 400 volts. The shadow mask 25 is inserted into panel 12 (FIG. 1) while the positively charged photoconductor is exposed, through the shadow mask, to light from a xenon flash lamp 38 arranged within a conventional three-in-one "headlight" (represented by lens 40 in FIG. 3c). After each exposure, the lamp is moved to a different position to duplicate the angle of incidence of the electron beams emitted by the electron gun. It should be noted that three exposures from three different lamp positions are required to discharge the areas of the photoconductor where the light-emitting phosphors will later be deposited to form the screen. After the exposure step, the shadow mask 25 is removed from the panel 12 and the panel is moved to a first developer 42 (Figure 3d). The first developer contains appropriately prepared dry-sprayed particles of a material having the structure required to form the light-absorbent black matrix screen and surface-treated insulating support beads (not shown) of approximately 100 to approximately 300 micrometers in diameter, which impart a triboelectric charge to the particles of the black matrix material, as described below.

Suitable black matrix materials generally contain black pigments that are stable at a tube processing temperature of 450°C. Suitable black pigments for use in the manufacture of matrix materials include iron-manganese oxide, iron-cobalt oxide, zinc-iron sulfide, and insulating carbon black. The black matrix material is prepared by mixing, in a molten state, the pigment, a polymer, and an appropriate charge control agent that controls the magnitude of the triboelectric charge imparted to the matrix material. The material is ground to an average particle size of approximately 5 micrometers.

The black matrix material and the surface-treated support beads are mixed in developer 42 using approximately one to two weight percent of the black matrix material. The materials are mixed such that the finely divided matrix particles can contact and become charged, such as negatively, by the surface-treated support beads. The negatively charged matrix particles are expelled from developer 42 and attracted to the positively charged, unexposed areas of the photoconductive layer 34 to directly develop that area. It should be noted that infrared radiation is used to bond the matrix material by fusing, or thermally bonding, the polymer component of the matrix material to the photoconductive layer to form the matrix 23 shown in FIGS. 2 and 3e.

The photoconductive layer 34 containing the matrix 23 is uniformly charged to a positive potential of approximately 200 to 400 volts by applying the first of three screen-forming materials, dry-sprayed phosphors constituting the color-emitting materials. The shadow mask 25 (Figure 3c) is reinserted into the panel 12, while the selective areas of the photoconductive layer 34 corresponding to the positions where the green-emitting phosphor will be deposited are exposed to visible light from a first position within the luminaire to selectively discharge the exposed areas. The first light position approximates the angle of convergence of the electron beam incident on the green-emitting phosphor. Shadow mask 25 is then removed from panel 12, and the panel is moved to a second developer 42 containing appropriately prepared dry-sprayed particles of a structural screen material, comprising the green-emitting phosphor, and surface-treated carrier beads. The phosphor particles are surface-treated with an appropriate charge control material, as described below. One thousand grams of surface-treated carrier beads are combined with 15-25 grams of surface-treated phosphor particles in the second developer 42. The carrier beads are treated to impart a certain charge, such as a positive charge, to the phosphor particles. The positively charged, green-emitting phosphor particles are ejected from the developer, repelled by the positively charged areas of the photoconductive layer 34 and matrix 23, and deposited onto the light-exposed, i.e., discharged areas of the photoconductive layer, in a process known as "backward development." The deposited green-emitting phosphor particles are attached to the photoconductive layer by exposing the surface-treated phosphor to infrared radiation, which causes the phosphor to melt, or thermally bond, to the photoconductive layer.

The charging, exposure, development, and fixing processes are repeated for the surface-treated, dry-sputtered, blue-emitting, and red-emitting phosphor particles, which constitute the structural material particles for the screen. Exposure to visible light to selectively discharge the positively charged areas of the photoconductive layer 34 is achieved from a second and then a third position within the illumination apparatus, so as to approximate the convergence angles of the electron beams incident on the blue-emitting and red-emitting phosphors, respectively. The dry-sputtered, positively triboelectrically charged phosphor particles are mixed with the surface-treated carrier beads in the ratio previously indicated and are then ejected from a third developer 42 and subsequently from a fourth developer 42, repelled from the positively charged areas of the previously deposited screen materials, and are then deposited onto the discharged areas of the photoconductive layer 34 to yield the blue-light-emitting and red-light-emitting phosphor elements, respectively.

In the previously described version, the initial surface treatment step includes the formation of a continuous coating of silicon dioxide (silica) on the surface of each of the phosphor particles, e.g. blue-luminescent (ZnS/Ag), green-luminescent (ZnS/Cu, Au, Al) and red-luminescent (Y2O2S/Eu).

EXAMPLE 1

To form this coating, 6.6 grams of a colloidal silica sol, commercially marketed as NYCOL 2030 EC (available from PQ Corporation, Ashland, MA), is dissolved in 1 liter of isopropanol. One kilogram of a blue-luminescent phosphor, such as ZnS/Ag, is added to the solution, followed by stirring for two hours to completely disperse the phosphorus particles. The resulting phosphorus particles, continuously coated with silicon dioxide (silica), are dried in a rotary evaporator at 85°C until all the solvent is removed from the mixture. Dried, silica-coated phosphorus and a virgin, uncoated phosphorus were tested for charge-to-mass ratio and shield weight by mixing 3 grams of phosphorus with 150 grams of surface-treated fluorosilane support beads. The fluorosilane-treated beads are triboelectrically negative and therefore induce a positive charge on the phosphor particles. The test procedure is described herein, while the results for the virgin phosphorus (blue Z936) and the silica-coated phosphorus (Example 1) are reported in Table 1. The silica coating described above is also applied to the green luminescent phosphorus (ZnS/Cu,Au,Al ) and the red luminescent core phosphorus (Y2O2S/Eu) using the process steps described above. An alternative colloidal silica sol that may be used is commercially available under the registered trademark Cab-O-Sperse Grade B (marketed by Cabot Corporation, Tuscola, Illinois U.S.A.).

The silica coating on the phosphor particles provides a hydroxy functional group in the form of silanol. Silane or titanate coupling agents react with the silanol groups to form covalent chemical bonds. Phosphors initially treated to obtain a silica coating and subsequently overcoated using a silane or titanate coupling agent exhibit a surface with an organic functional group, as determined by the coupling agent used for the overcoating. These organic groups of surface-treated phosphors react with the functional groups formed on the carrier beads, as described in the previously identified U.S. Patent Application No. 287,357, to determine the magnitude of the triboelectric charge on the surface-treated phosphor particles.

EXAMPLE 2

One-tenth (0.1) gram of N(2-aminoethyl-3-aminopropyl)methyl dimethoxysilane (amino No. 1) is dissolved in 200 ml of isopropanol to form a coating solution. One hundred grams of silica-coated, blue-luminescent phosphor particles obtained according to the process of Example 1 are added to the coating solution and stirred ultrasonically for approximately 10 minutes. The surface-treated, aminosilane-coated, blue-luminescent phosphor is dried in a rotary evaporator. The dried phosphor is then sifted through a 400-mesh sieve.

Three (3) grams of a dry powdered, surface-treated, blue-luminescent aminosilane phosphor were mixed with approximately 100 grams of surface-treated fluorosilane carrier beads. The fluorosilane-treated beads were triboelectrically negative and therefore induced a positive charge on the aminosilane-treated blue-luminescent phosphor particles. The charge-to-mass ratio and electrophotographic screen (EPS) characteristics (screen weight) of the phosphor fabricated using this process were evaluated as previously described, and the corresponding results are shown in Table 1.

EXAMPLE 3

Similar to Example 2, except that N-(aminoethyl aminopropyl) triethoxysilane (amino No. 2) replaces amino compound No. 1. All other materials and all process steps remain unchanged. The test results are reported in Table 1.

EXAMPLE 4

Similar to Example 2, except that 3-(aminopropyl) dimethylethoxysilane (amino No. 3) replaces compound amino No. 1. All other materials and process steps remain unchanged. Test results are shown in Table 1.

EXAMPLE 5

Similar to Example 2, except that (aminopropyl)triethoxysilane (amino No. 4) replaces the amino compound No. 1. All other materials and process steps remain unchanged. Test results are reported in Table 1.

EXAMPLE 6

Similar to Example 2, except that (methacryloxypropyl)triethoxysilane (amino compound No. 6) replaces amino compound No. 1. All other materials and process steps remain unchanged. Test results are shown in Table 1.

EXAMPLE 7

One-tenth (0.1) gram of isopropyl tri(dioctyl pyrophosphate) titanate (Titanate) is dissolved in 200 ml of a 50:50 mixture of isopropanol and heptane to form a coating solution. One hundred grams of silica-coated, blue-luminescent phosphor particles (from Example 1) are added to the coating solution, followed by stirring for two hours. The titanate-coated, blue-luminescent phosphor is dried in a rotary evaporator, and the dried phosphor is then sifted through a 400-mesh sieve. The test procedure is described later in this paper, and the results are shown in Table 1.

EXAMPLE 8

One-tenth (0.1) gram of amino No. 1 is dissolved in 200 ml of isopropanol, as described in Example 1, to form a coating solution. One hundred grams of silica-coated, green luminescent phosphor particles are added to the coating solution, followed by stirring for approximately two hours. The green luminescent phosphor, surface-treated with aminosilate, is dried in a rotary evaporator and then sifted through a 400-mesh sieve.

Three (3) grams of the dry, powdered, aminosilane-coated, green-luminescent phosphor material are mixed with 100 grams of fluorosilane-coated carrier beads and tested as previously described. The test results are shown in Table 2. A virgin, green-luminescent phosphor (ZnS/Cu,Au,Al) with no silica coating and no aminosilane coating is used as a control phosphor for the green-luminescent phosphors in Table 2.

EXAMPLE 9

Similar to Example 8, except that amino No. 2 replaces amino No. 1. All other materials and process steps remain unchanged. Test results are shown in Table 2.

EXAMPLE 10

Similar to Example 8, except that amino No. 3 replaces amino No. 1. All other materials and process steps remain unchanged. Test results are shown in Table 2.

EXAMPLE 11

Similar to Example 8, except that amino No. 4 replaces amino No. 1. All other materials and process steps remain unchanged. Test results are shown in Table 2.

EXAMPLE 12

One tenth (0.1) of a gram, of amino No.

1 is dissolved in 200 ml of isopropanol to form a coating solution. One hundred grams of silica-coated red phosphorus particles (Y2O2S/Eu) are added to the coating solution, followed by stirring for approximately two hours. The red-luminescent phosphorus material, surface-treated with aminosilane, is dried in a rotary evaporator, and then sifted through a 400-mesh sieve.

Three (3) grams of the dry powdered, aminosilane-coated, red-luminescent phosphor material were mixed with 100 grams of fluorosilane-coated carrier beads and tested as previously described. The test results were shown in Table 2. A virgin, red-luminescent phosphor (Y2O2S/Eu), having neither a silica coating nor an aminosilane coating, was used as a control for the red-luminescent phosphors in Table 2.

EXAMPLE 13

Similar to Example 12, except that amino No. 2 replaces amino No. 1. All other materials and all process steps remain unchanged. The test results are reported in Table 2.

EXAMPLE 14

Similar to Example 12, except that amino No. 3 replaces amino No. 1. All other materials and process steps remain unchanged. The test results are reported in Table 2.

EXAMPLE 15

Similar to Example 12, except that amino No. 4 replaces amino No. 1. All other materials and process steps remain unchanged. Test results are shown in Table 2.

TABLE 1

Virgin, blue-luminescent phosphors and silane-treated, blue-luminescent phosphors in contact with fluorosilane-treated glass beads for positively charged phosphors.

TABLE 2

Virgin, green-luminescent, red-luminescent phosphors and silane-treated, green-luminescent phosphors in contact with fluorosilane-treated glass beads for positively charged phosphors.

Test results were determined using a test panel (not shown) consisting of an insulated board with a metal conductor laminated on each major surface, with a centrally located opening extending across the major surfaces of the board and the conductors. Preferably, the opening is 2.54 cm in diameter. A metal mesh of approximately 50 to 100 mesh extends across the opening and is connected to one of the metal conductors. A glass plate coated with TIC extends across the opening and is placed on the other metal conductor, such that the TIC coating is in contact with it. To measure the positively charged phosphor particles, a potential ranging from 100 to 600 volts is applied to the conductor connected to the metal mesh, with the conductor contacting the TIC coating connected to ground. The potential difference between the mesh and the glass is approximately 10<3 >V/cm. The test panel is positioned approximately 7.62 cm above a developer containing the surface-treated phosphor and carrier beads as previously described in Examples 1, 2, 7, 8, and 12. The developer is closed at one end with a mesh suitable for the passage of the finely divided phosphor particles but not the carrier beads. An air jet (velocity approximately 104 cm/sec) separates the phosphor particles from the carrier beads and permits the expulsion of the charged phosphor particles, represented in this case by positively charged particles, from the developer onto the metal mesh and the TIC-coated glass plate. The resulting electrostatic charge on the TIC-coated glass plate is measured using an electrometer, and the mass of the phosphor particles is determined by weighing the glass plate before and after the test. The quotient of these measurements represents the average triboelectric charge to mass ratio. The deposition area of the TIC-coated glass plate is known and controlled by the size of the aperture in the test panel. The test results are summarized in Tables 1 and 2. In each case, the surface-treated glass beads include a fluorosilane coating to impart a positive charge to the phosphor particles. A control was conducted for each chromatic phosphor tested. The control phosphors were not surface-treated. The results showed that the surface-treated phosphors had a much higher charge-to-mass ratio than the untreated phosphors and were therefore less likely to be positively charged. It has been shown that the screen weights for surface-treated phosphors are substantially higher than those for untreated phosphors. The best results were obtained by superficially treating the phosphors with N-(aminoethyl aminopropyl)triethoxysilane or N(2-aminoethyl-3-aminopropyl) methyldimethoxysilane.

Claims (5)

1. A method of surface treatment of dry powdered phosphor particles for use in the manufacture of a display screen for a cathode ray tube, to control the triboelectric charge characteristics of said phosphor particles, characterised in that it involves the following steps consisting of: provide said phosphor particles with a first silica coating, dissolve a coupling agent selected from the group consisting of silane and titanates, in an appropriate solvent, to form a mixture, superficially treating said silica-coated phosphor particles with said mixture, so as to obtain a second coating of said coupling agent on top of the same, filter the superficially treated phosphorus particles, rinse the filtered, surface-treated particles in said solvent, and dry said surface-treated phosphor particles. 2. The method of claim 1, characterized in that said mixture is formed by dissolving 0.1 grams of a silane selected from the group consisting of: N(2-aminoethyl-3-aminopropyl ) methyl dimethoxysilane, N-(aminoethyl aminopropyl) triethoxysilane, 3 -(aminopropyl ) dimethyl ethoxysilane, (aminopropyl) triethoxysilane, (methacryloxypropyl-trimethoxysilane, (n-decyl)methyldichlorosilane, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)~1-dimethylchlorosilane and (tridecafluoro-1,1,2,2-tetrahydrodecyl)-1-dimethyltriethoxysilane, in 200 ml of isopropanol. 3. The method of claim 2, characterized in that said surface coating step includes the following steps; (i) add 100 grams of said silica-coated particles to said mixture, and {ii) mix, ultrasonically, said silica-coated particles and said mixture, for 10 minutes. 4. A cathode ray tube comprising an electrophotographically fabricated luminescent viewing screen and means for selectively exciting areas of said screen to luminescence, said screen comprising a layer of dry-sputtered phosphor particles capable of emitting light in a particular portion of the visible spectrum, characterised in that said dry-sputtered phosphor particles have a first coating of silica thereon and a second coating of a coupling agent overlying said first coating for controlling the surface charge of said particles during the manufacture of said screen (22). 5. A cathode ray tube comprising an electrophotographically fabricated luminescent viewing screen and means for selectively exciting areas of said screen to luminescence, said screen comprising a layer of dry-sputtered phosphor particles capable of emitting light in a particular portion of the visible spectrum, characterised in that said dry-sputtered phosphor particles have a first coating of silica thereon and a second coating of a coupling agent overlying said first coating for controlling the surface charge of said particles during the manufacture of said screen (22), said coupling agent being selected from the group consisting of N(2-aminoethyl-3-aminopropyl)methyldimethoxysilane, (aminopropyl)triethoxysilane, 3-(aminopropyl)dimethylethoxysilane, N-{aminoethyl-3-aminopropyl)triethoxysilane, (methacryloxypropyl) trimethoxy silane, isopropyl-tri(dioctyl-pyrophosphate)titanate, (n-decyl) methyl dichlorosilane, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)-i-dimethylchlorosilane and (tridecafluoro 1,1,2,2-tetraidooctyl)~1-dimethyl-triethoxysilane.